Hologram recording apparatuses so far developed include the one disclosed in patent document 1 below. The hologram recording apparatus according to this document records and reproduces the hologram on and from a hologram recording medium, based on what is known as a coaxial system. The hologram recording apparatus is configured so as to emit a reference beam and an recording beam (signal beam) to the hologram recording medium through a same objective lens in the recording process, and to receive a reconstruction beam through a condenser lens located opposite to the objective lens, in the reproducing process.
As illustrated in FIG. 6, an objective lens 700 is normally configured so as to form a telecentric system. The telecentric system is an optical system in which either an entrance pupil or an exit pupil is assumed to be at infinite distance, so that a beam flux is barely converged and runs maintaining a generally constant diameter. A condenser lens which is not illustrated also forms the telecentric system like the objective lens 700. In the case where, for example, a spatial light modulator 500 is located at a position corresponding to the exit pupil; an entrance diameter of an on-axis beam flux S1 and an off-axis beam flux S2 at an entrance surface of the objective lens 700 is denoted by D; and a diameter of a beam waist C formed by the on-axis beam flux S1 and the off-axis beam flux S2 at a position corresponding to the entrance pupil is denoted by d, the entrance diameter D becomes equal to the diameter d of the beam waist C, in the telecentric system. The diameter d of the beam waist C (entrance diameter D of the on-axis beam flux and the off-axis beam flux) can be expressed as d=2F·NA, wherein NA represents the object-side numerical aperture, and F the focal length. The smaller the diameter d of the beam waist C is, the smaller a unit area for recording the hologram can be made. Accordingly, a hologram recording medium B is located at the position of the entrance pupil, where the beam waist C is formed (Ref. FIG. 7).
Patent Document 1: Japanese Laid-open Patent Publication No. 2006-113296
However as illustrated in FIG. 7, with reference to the off-axis beam fluxes S2, S3 of different field angles, when the off-axis beam fluxes S2, S3 reach a recording layer B2 through a cover layer B1 of the hologram recording medium B, the off-axis beam fluxes S2, S3 define different angles with respect to an optical axis Lx according to the refractive index of the cover layer B1, and hence on-axis focal points L2p, L3p, at the respective intersection of the principal ray L2, L3 of the off-axis beam fluxes S2, S3 and the optical axis Lx, are shifted from each other. In this case, the beam waist C formed in the hologram recording medium B is expanded, resulting in the drawback that the unit area for recording the hologram cannot be made sufficiently small.